The present invention relates to a ceramic honeycomb catalyst carrier used for purifying an exhaust gas from automobile and so on and a method of producing the same.
A ceramic honeycomb catalyst carrier is used for fixing a catalyst for purifying an exhaust gas from automobile and so on. Generally, the ceramic honeycomb catalyst carrier has a plurality of partition walls each having a number of pores, on which xcex3-alumina having a high specific surface area and a catalyst are coated, thereby being used for automobile and so on. In view of a recent beef-up of exhaust gas regulation, it is required to make a thickness of the partition wall of the ceramic honeycomb catalyst carrier thin so as to improve a catalyst performance and an engine performance.
However, if the partition wall of the ceramic honeycomb catalyst carrier is made thin as mentioned above, a mechanical strength of the ceramic honeycomb catalyst carrier is decreased. In order to eliminate such a problem, in Japanese Patent Laid-Open Publication No. 10-174885 (JP-A-10-174885) for example, a technique, wherein a dense cordierite having a high strength and a porosity of 18% or less is obtained by controlling a raw material size of talc and an amount of CaO, is disclosed. However, if a porosity is decreased, a strength can be improved, but there arises a new problem such that a coatability of xcex3-alumina having a high specific surface area and catalyst is deteriorated.
An object of the invention is to eliminate the drawbacks mentioned above and to provide a ceramic honeycomb catalyst carrier having a sufficient strength as a catalyst carrier and excellent catalyst coatability even if a partition wall is thin and a method of producing the same.
According to the invention, a ceramic honeycomb catalyst carrier is characterized in that a porosity is 20% or less and an average surface roughness (Ra) of a partition wall of the carrier is 0.51 xcexcm or more.
Moreover, according to a first aspect of the invention regarding a producing method, a method of producing the ceramic honeycomb catalyst carrier mentioned above, comprises the steps of: forming ceramic raw materials to obtain a ceramic honeycomb formed body; drying the ceramic honeycomb formed body to obtain a ceramic honeycomb dried-up body; roughening a surface of the partition wall of the ceramic honeycomb dried-up body by exposing the ceramic honeycomb dried-up body in an airflow in which polishing powders are included; and sintering the ceramic honeycomb dried-up body after a surface treatment.
Further, according to a second aspect of the invention regarding a producing method, a method of producing the ceramic honeycomb catalyst carrier mentioned above, comprises the steps of: forming ceramic raw materials to obtain a ceramic honeycomb formed body; drying and sintering the ceramic honeycomb formed body to obtain a ceramic honeycomb sintered body; and roughening a surface of the partition wall of the ceramic honeycomb sintered body by exposing the ceramic honeycomb sintered body in an airflow or a water flow in which polishing powders are included.
In the present invention, a ceramic honeycomb catalyst carrier, in which a porosity is 20% or less and a partition wall of the carrier is roughened so as to be an average surface roughness (Ra) of 0.5 1 xcexcm or more, can be obtained by performing a surface treatment with respect to a surface of the partition wall preferably by means of polishing powders such as alumina and diamond after forming and drying ceramic raw materials or further after sintering a formed body. In this manner, it was found that it is possible to obtain a ceramic honeycomb catalyst carrier having a high strength and excellent xcex3-alumina or catalyst coatability.
In the present invention, the reason for defining an average surface roughness as 0.5 1 xcexcm or more is that, if it is less than 0.5xcexcm, xcex3-alumina and catalyst are not sufficiently coated thereon. Moreover, the reason for defining porosity as 20% or less is that, if it is larger than 20%, a sufficient strength can not be obtained. Further, the surface treatment mentioned above may be performed by circulating a liquid, in which polishing powders such as alumina and diamond are dispersed, by means of a pump, or, by exposing a ceramic honeycomb for a definite period of time in an atmosphere in which polishing powders are included in an airflow. Furthermore, as a material of the ceramic honeycomb catalyst carrier, use may be made of a popular ceramics such as partially stabilized zirconia, silicon nitride, alumina, silicon carbide, and zirconium phosphate, other than cordierite.